The present application claims priority of Japanese Patent Application No. 2000-232953 filed on Aug. 1, 2000, the entire contents thereof are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an improvement in a hydrostatic continuously variable transmission including a combination of a swash plate type hydraulic pump and a swash plate type hydraulic motor.
2. Description of Background Art
A hydrostatic continuously variable transmission including a swash plate type hydraulic pump, a swash plate type hydraulic motor, and a hydraulic closed circuit disposed between the hydraulic pump and the hydraulic motor, wherein the hydraulic pump is integrally combined with the hydraulic motor by an output shaft press-fitted in a cylinder block in which a pump cylinder bore and a motor cylinder bore are made in parallel to the output shaft, is disclosed, for example, in Japanese Unexamined Patent Publication No. 303525/1997. In this kind of conventional hydrostatic continuously variable transmission, the cylinder bore (piston chamber) is deformed at the directly outside portion where the output shaft is press-fitted into the cylinder block by the effect of press-fitting the output shaft to make it impossible to keep its circular cross section As a result, this inevitably enlarges the inner diameter of the cylinder bore and hence increases the gap between a piston and the inner wall of the cylinder bore, which presents a problem of increasing oil leakage and reducing efficiency.
In order to solve the above-mentioned problem presented by the conventional hydrostatic continuously variable transmission, the present invention proposes a hydrostatic continuously variable transmission including a hydraulic pump, a hydraulic motor, and a hydraulic closed circuit disposed between the hydraulic pump and the hydraulic motor, wherein the hydraulic pump is integrally combined with the hydraulic motor by an output shaft press-fitted in a cylinder block in which pump cylinder bores and motor cylinder bores are made in parallel to the output shaft, and characterized in that the diameters of the back portions of the pump cylinder bore and the motor cylinder bore are larger than the diameters of their entry portions and that a press-fitting portion is not provided at the contact portions with the output shaft inside the entry portions thereof.
Since the present invention is constituted as described above and the diameters of the back portions of the pump cylinder bore and the motor cylinder bore are made larger than the diameters of their entry portions, even if the cylinder bore is deformed by press-fitting the output shaft, the interference between the cylinder bore and the piston can therefore be prevented.
Since a press-fitting portion is not made at the contact portions with the output shaft inside the entry portions of the cylinder bores, the entry portions of the cylinder bores are not deformed. Therefore, the inner diameters of the entry portions can be determined so that the cylinder bores can maintain suitable clearances with respect to a pump piston and a motor piston. As a result, this can prevent an excessive leakage of oil and a decrease in efficiency.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.